1. Field of Invention
This invention relates to a switching circuit and more particularly a temperature compensated emitter coupled logic gate.
2. Description of Prior Art
Many circuits exist for temperature and voltage compensation of basic logic circuits. In an emitter coupled logic gate, the output logic signal is generated at the emitter terminal of an emitter-follower output transistor. Without temperature compensation this output logic level varies due to the negative temperature coefficient associated with the base-to-emitter junction of the transistor. That is, as the temperature increases, the base-to-emitter voltage drop, V.sub.BE, decreases, and conversely with a decrease in temperature the V.sub.BE increases. Since the noise immunity of any logic gate is related to the stability of the output logic level, any variations of the output logic signal due to temperature changes deleteriously affects the noise margin or noise immunity of the switching circuit.
Compensation circuits using back-to-back diodes and current sources have been suggested for improving the performance of ECL gates. However, this solution limits the overall speed of the logic gate due to the requirement of charging certain collector nodes in the circuit, and furthermore, certain rounding of the DC transfer and power supply characteristic curves are experienced thus reducing the noise immunity or noise margin of the circuit.
Also, some compensating circuits employ a plurality of current sources which are always connected to collector node associated with either the reference transistor or the input switching transistors in order to compensate for output voltage variations due to changes in the base-to-emitter drops associated with the emitter-follower output transistor during ambient temperature changes. However, these ECL gates suffer in their switching speed response in that the two collector nodes additional stray capacitance must be charged every time the logic gate switches states. Moreover, these prior art solutions require a greater number of components and consume a commensurate greater amount of power.